1. Field of Invention
The present invention relates to a method for measuring a to-be-measured device. More particularly, the present invention relates to a method of establishing an optical critical dimension module and a measuring method of using the optical critical dimension module.
2. Description of Related Art
Along with the development of semiconductor fabrication techniques, the level of integration of integrated circuits increases continuously and the critical dimensions correspondingly decrease. In order to achieve a high level of integration and high efficiency, the critical dimension of each characteristic feature, the profile and the degree of uniformity must be precisely controlled.
In the current semiconductor fabrication process, the characteristic feature is formed by a transferring of patterns via the photolithograph and etching techniques. A typical lithograph process includes a coating of a photoresist, a soft baking process, an exposure process, a development process and a hard baking process. The accuracy of a characteristic feature highly relates to the quality of the photoresist pattern. Hence, to ensure the quality of a photoresist pattern and to mitigate any process problems, an to ensure the quality of a photoresist pattern and to mitigate any process problems, an ADI (After Development Inspection) is normally performed subsequent to a development process. If after being inspected, the photoresist pattern is determined to contain defects, the photoresist needs to be removed and the fabrication process has to be re-conducted. If the photoresist pattern contains undetected defects, the etching performed in the subsequent etching process may be inaccurate, leading to irreparable errors and an ultimate disposal of the wafer.
The optical critical dimension measuring method, such as the spectrum critical dimension (SCD) measuring method, is applicable in the inspection of a photoresist pattern after a development process. According to the optical critical dimension method, the test key spectrum that corresponds to the photoresist pattern is being measured first to obtain the real curve. Thereafter, an appropriate module is obtained from a library, and a correlation process is performed to correlate the real curve of the spectrum with a theoretical curve. Based on the corresponding parameters of the theoretical curve, relevant information of the to-be-measured photoresist pattern is acquired.
Since a spectrum is obtained from an optical critical dimension method, the characteristics of each film layer underneath the photoresist pattern may influence the behavior of the spectrum curve. Hence, during the construction of the modules of the library, the corresponding film file of each film layer that is underneath the photoresist pattern is obtained to establish a corresponding stacked film.
In the fabrication of a gate, the photoresist pattern covers the polysilicon layer and the gate oxide layer. Accordingly, when the optical critical dimension measuring method is used to measure a photoresist pattern, a module that includes a polysilicon and silicon oxide stacked layer is selected from the library to perform the correlation. Further, during the correlation, the thickness and the optical parameters, such as refraction index or extinction coefficient of each material layer are defined at certain fixed values. However, with the current approach, the theoretical curve and the real curve of the spectrum are unable to correlate. The corresponding parameters of the theoretical curve do not accurately reflect the actual conditions of the photoresist pattern. Hence, factual information regarding the photoresist pattern is unable to obtain.